Conventionally, in starting a diesel engine or the like, a sheathed heater in which a coil for generating heat embedded in insulating powder is provided in a bottomed cylindrical metallic sheath has been used. However, in the sheathed heater, since the coil for generating heat is embedded in insulating powder, thermal conductivity is low and a long period of time is needed for raising temperature. Therefore, in recent years, a ceramic heater which enhances thermal conductivity and is capable of rapid temperature rise through a structure embedding a heat-generating resistor comprising, as major components, an electrically conductive ceramic, such as tungsten carbide or molybdenum disilicide, and silicon nitride, in a base substance comprising insulative silicon nitride ceramic and which is excellent in corrosion resistance at high temperature has been developed. This ceramic heater is particularly used in a glow plug or the like in which temperature goes up to 1200° C. or more.
When a heat-generating resistor of the ceramic heater is produced, a rare earth oxide is added as a sintering additive to the electrically conductive ceramic and silicon nitride to form a grain boundary between an electrically conductive ceramic crystal phase and a silicon nitride crystal phase. When a glass phase having a low melting point is present in this grain boundary, durability and other properties of the ceramic heater are deteriorated. Then, ordinarily, a crystal phase such as a disilicate crystal phase (RE2Si2O7; RE representing a rare earth element) or monosilicate crystal phase (RE2SiO5) is precipitated (for example, refer to JP-A No. 11-214124).
However, it is difficult to uniformly precipitate the crystal phase over the entire grain boundary of the heat-generating resistor and, accordingly, the crystal phase is precipitated only in a portion of the grain boundary and a component which did not contribute to crystallization remains as a glass phase. Namely, the grain boundary comes to have a locally uneven crystal structure. As a result, there are cases in which, when the ceramic heater was supplied with current, an electric conduction defect is generated in the heat-generating resistor, a rise of a resistance value of the heat-generating resistor easily occurs, and it comes to be impossible to raise the temperature up to a predetermined level.
An object of the present invention is to solve the conventional problems and to provide a ceramic heater which prevents electric conduction defects of a heat-generating resistor caused by supplied current, and is excellent in voltage endurance.